doc: Formatting fixes for readme.rst

The readme.rst file in the project root is the front-page that
is displayed on Github and if viewing the TF-A repository on
git.trustedfirmware.org in the "about" view. It now contains a
small amount of stub content, and directs readers to the
ReadTheDocs documentation via trustedfirmware.org/docs/tf-a.

The Github renderer is displaying the content fine but the cgit
viewer displays some "backlink" errors because some content
substitutions were left in place (terms surrounded by pipe
symbols), e.g. |TF-A|.

This patch removes those substitutions, that are not supported
by cgit, and also updates one heading to clarify where to find
the new docs.

Change-Id: I358451df45b8c99975ba0b6db8ea61253a10560d
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

readme.rst

 Trusted Firmware-A
 ==================
 
-Trusted Firmware-A is a reference implementation of secure world software for
-`Arm A-Profile architectures`_ (Armv8-A and Armv7-A), including an
-Exception Level 3 (EL3)`Secure Monitor`_. It provides a suitable starting point
-for productization of secure world boot and runtime firmware, in either the
-|AArch32| or |AArch64| execution states.
+Trusted Firmware-A (TF-A) is a reference implementation of secure world software
+for `Arm A-Profile architectures`_ (Armv8-A and Armv7-A), including an Exception
+Level 3 (EL3) `Secure Monitor`_. It provides a suitable starting point for
+productization of secure world boot and runtime firmware, in either the AArch32
+or AArch64 execution states.
 
-|TF-A| implements Arm interface standards, including:
+TF-A implements Arm interface standards, including:
 
 -  `Power State Coordination Interface (PSCI)`_
 -  `Trusted Board Boot Requirements CLIENT (TBBR-CLIENT)`_
@@ -18,15 +18,15 @@ for productization of secure world boot and runtime firmware, in either the
 The code is designed to be portable and reusable across hardware platforms and
 software models that are based on the Armv8-A and Armv7-A architectures.
 
-In collaboration with interested parties, we will continue to enhance |TF-A|
+In collaboration with interested parties, we will continue to enhance TF-A
 with reference implementations of Arm standards to benefit developers working
 with Armv7-A and Armv8-A TrustZone technology.
 
 Users are encouraged to do their own security validation, including penetration
 testing, on any secure world code derived from TF-A.
 
-Read More
----------
+More Info and Documentation
+---------------------------
 
 To find out more about Trusted Firmware-A, please `view the full documentation`_
 that is available through `trustedfirmware.org`_.